The polyester fabrics and articles which are in wide use today are produced from a polymer of ethylene glycol and teraphthalic acid. Teraphthalic acid is produced by the oxidation of para-xylene. Para-xylene is typically recovered from a predominantly C.sub.8 aromatic hydrocarbon fraction derived from various sources such as catalytic reforming by liquid-liquid extraction and/or fractional distillation. The para-xylene is commercially separated from a para-xylene-containing feed stream, usually containing all three xylene isomers, by either crystallization or adsorptive separation or a combination of these two techniques. Adsorptive separation is the newer technique and has captured the great majority of the market share of newly constructed plants for the production of para-xylene.
Essentially all of these adsorptive separation units use a simulated countercurrent movement of the adsorbent and the xylene containing feed stream. This simulation is performed using established commercial technology wherein the adsorbent is held in place in one or more cylindrical adsorbent chambers and the positions at which the streams involved in the process enter and leave the chambers are slowly shifted along the length of the beds. Normally there are at least four streams (feed, desorbent, extract and raffinate) employed in this procedure and the location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneous shifted in the same direction at set intervals. Each shift in location of these transfer points delivers or removes liquid from a different bed within the chamber. This shifting could be performed using a dedicated line for each stream at the entrance to each bed. However, this will greatly increase the cost of the process and therefore the lines are reused and each line carries one of the four process streams at some point in the cycle.